Add size comparison methods to IterableOps

Add IterableOps.sizeCompare(Int), .sizeCompare(Iterable[_]), and
.sizeIs

src/library/scala/collection/IndexedSeq.scala

@@ -52,10 +52,16 @@ trait IndexedSeqOps[+A, +CC[_], +C] extends Any with SeqOps[A, CC, C] { self =>
 
   override def last: A = apply(length - 1)
 
-  override def lengthCompare(len: Int): Int = Integer.compare(length, len)
+  override final def lengthCompare(len: Int): Int = Integer.compare(length, len)
 
   final override def knownSize: Int = length
 
+  override final def sizeCompare(that: Iterable[_]): Int = {
+    val res = that.sizeCompare(length)
+    // can't just invert the result, because `-Int.MinValue == Int.MinValue`
+    if (res == Int.MinValue) 1 else -res
+  }
+
   override def search[B >: A](elem: B)(implicit ord: Ordering[B]): SearchResult =
     binarySearch(elem, 0, length)(ord)
 

src/library/scala/collection/Iterable.scala

@@ -228,6 +228,88 @@ trait IterableOps[+A, +CC[_], +C] extends Any with IterableOnce[A] with Iterable
   /** A view over the elements of this collection. */
   def view: View[A] = View.fromIteratorProvider(() => iterator)
 
+  /** Compares the size of this $coll to a test value.
+    *
+    *   @param   otherSize the test value that gets compared with the size.
+    *   @return  A value `x` where
+    *   {{{
+    *        x <  0       if this.size <  otherSize
+    *        x == 0       if this.size == otherSize
+    *        x >  0       if this.size >  otherSize
+    *   }}}
+    *  The method as implemented here does not call `size` directly; its running time
+    *  is `O(size min _size)` instead of `O(size)`. The method should be overwritten
+    *  if computing `size` is cheap.
+    */
+  def sizeCompare(otherSize: Int): Int = {
+    if (otherSize < 0) 1
+    else {
+      val known = knownSize
+      if (known >= 0) Integer.compare(known, otherSize)
+      else {
+        var i = 0
+        val it = iterator
+        while (it.hasNext) {
+          if (i == otherSize) return if (it.hasNext) 1 else 0
+          it.next()
+          i += 1
+        }
+        i - otherSize
+      }
+    }
+  }
+
+  /** Returns a value class containing operations for comparing the size of this $coll to a test value.
+    *
+    * These operations are implemented in terms of [[sizeCompare(Int) `sizeCompare(Int)`]], and
+    * allow the following more readable usages:
+    *
+    * {{{
+    * this.sizeIs < size     // this.sizeCompare(size) < 0
+    * this.sizeIs <= size    // this.sizeCompare(size) <= 0
+    * this.sizeIs == size    // this.sizeCompare(size) == 0
+    * this.sizeIs != size    // this.sizeCompare(size) != 0
+    * this.sizeIs >= size    // this.sizeCompare(size) >= 0
+    * this.sizeIs > size     // this.sizeCompare(size) > 0
+    * }}}
+    */
+  @inline final def sizeIs: IterableOps.SizeCompareOps = new IterableOps.SizeCompareOps(this)
+
+  /** Compares the size of this $coll to the size of another `Iterable`.
+    *
+    *   @param   that the `Iterable` whose size is compared with this $coll's size.
+    *   {{{
+    *        x <  0       if this.size <  that.size
+    *        x == 0       if this.size == that.size
+    *        x >  0       if this.size >  that.size
+    *   }}}
+    *  The method as implemented here does not call `size` directly; its running time
+    *  is `O(this.size min that.size)` instead of `O(this.size + that.size)`.
+    *  The method should be overwritten if computing `size` is cheap.
+    */
+  def sizeCompare(that: Iterable[_]): Int = {
+    val thatKnownSize = that.knownSize
+
+    if (thatKnownSize >= 0) this sizeCompare thatKnownSize
+    else {
+      val thisKnownSize = this.knownSize
+
+      if (thisKnownSize >= 0) {
+        val res = that sizeCompare thisKnownSize
+        // can't just invert the result, because `-Int.MinValue == Int.MinValue`
+        if (res == Int.MinValue) 1 else -res
+      } else {
+        val thisIt = this.iterator
+        val thatIt = that.iterator
+        while (thisIt.hasNext && thatIt.hasNext) {
+          thisIt.next()
+          thatIt.next()
+        }
+        java.lang.Boolean.compare(thisIt.hasNext, thatIt.hasNext)
+      }
+    }
+  }
+
   /** A view over a slice of the elements of this collection. */
   @deprecated("Use .view.slice(from, until) instead of .view(from, until)", "2.13.0")
   @`inline` final def view(from: Int, until: Int): View[A] = view.slice(from, until)
@@ -690,6 +772,26 @@ trait IterableOps[+A, +CC[_], +C] extends Any with IterableOnce[A] with Iterable
 
 object IterableOps {
 
+  /** Operations for comparing the size of a collection to a test value.
+    *
+    * These operations are implemented in terms of
+    * [[scala.collection.IterableOps.sizeCompare(Int) `sizeCompare(Int)`]].
+    */
+  final class SizeCompareOps private[collection](val it: IterableOps[_, AnyConstr, _]) extends AnyVal {
+    /** Tests if the size of the collection is less than some value. */
+    @inline def <(size: Int): Boolean = it.sizeCompare(size) < 0
+    /** Tests if the size of the collection is less than or equal to some value. */
+    @inline def <=(size: Int): Boolean = it.sizeCompare(size) <= 0
+    /** Tests if the size of the collection is equal to some value. */
+    @inline def ==(size: Int): Boolean = it.sizeCompare(size) == 0
+    /** Tests if the size of the collection is not equal to some value. */
+    @inline def !=(size: Int): Boolean = it.sizeCompare(size) != 0
+    /** Tests if the size of the collection is greater than or equal to some value. */
+    @inline def >=(size: Int): Boolean = it.sizeCompare(size) >= 0
+    /** Tests if the size of the collection is greater than some value. */
+    @inline def >(size: Int): Boolean = it.sizeCompare(size) > 0
+  }
+
   /** A trait that contains just the `map`, `flatMap`, `foreach` and `withFilter` methods
     * of trait `Iterable`.
     *

src/library/scala/collection/Seq.scala

@@ -661,6 +661,8 @@ trait SeqOps[+A, +CC[_], +C] extends Any
     */
   def indices: Range = Range(0, length)
 
+  override final def sizeCompare(_size: Int): Int = lengthCompare(_size)
+
   /** Compares the length of this $coll to a test value.
     *
     *   @param   len   the test value that gets compared with the length.
@@ -674,23 +676,7 @@ trait SeqOps[+A, +CC[_], +C] extends Any
     *  is `O(length min len)` instead of `O(length)`. The method should be overwritten
     *  if computing `length` is cheap.
     */
-  def lengthCompare(len: Int): Int = {
-    if (len < 0) 1
-    else {
-      val known = knownSize
-      if (known >= 0) Integer.compare(known, len)
-      else {
-        var i = 0
-        val it = iterator
-        while (it.hasNext) {
-          if (i == len) return if (it.hasNext) 1 else 0
-          it.next()
-          i += 1
-        }
-        i - len
-      }
-    }
-  }
+  def lengthCompare(len: Int): Int = super.sizeCompare(len)
 
   /** Returns a value class containing operations for comparing the length of this $coll to a test value.
     *
@@ -706,7 +692,7 @@ trait SeqOps[+A, +CC[_], +C] extends Any
     * this.lengthIs > len     // this.lengthCompare(len) > 0
     * }}}
     */
-  @inline final def lengthIs: SeqOps.LengthCompareOps = new SeqOps.LengthCompareOps(this)
+  @inline final def lengthIs: IterableOps.SizeCompareOps = new IterableOps.SizeCompareOps(this)
 
   override def isEmpty: Boolean = lengthCompare(0) == 0
 

test/benchmarks/src/main/scala/scala/collection/LengthCompareOpsBenchmark.scala → test/benchmarks/src/main/scala/scala/collection/SizeCompareOpsBenchmark.scala

@@ -13,12 +13,12 @@ import scala.util.Random
 @Measurement(iterations = 10)
 @OutputTimeUnit(TimeUnit.NANOSECONDS)
 @State(Scope.Benchmark)
-class LengthCompareOpsBenchmark {
+class SizeCompareOpsBenchmark {
   @Param(Array("0", "1", "10", "100", "1000"))
   var size: Int = _
 
   @Param(Array("1", "100", "10000"))
-  var len: Int = _
+  var cmpTo: Int = _
 
   var values: List[Int] = _
 
@@ -27,11 +27,11 @@ class LengthCompareOpsBenchmark {
     values = List.fill(size)(Random.nextInt())
   }
 
-  @Benchmark def lengthCompareUgly: Any = {
-    values.lengthCompare(len) == 0
+  @Benchmark def sizeCompareUgly: Any = {
+    values.sizeCompare(cmpTo) == 0
   }
 
-  @Benchmark def lengthComparePretty: Any = {
-    values.lengthIs == len
+  @Benchmark def sizeComparePretty: Any = {
+    values.sizeIs == cmpTo
   }
 }

test/junit/scala/collection/IterableTest.scala

@@ -3,7 +3,9 @@ package scala.collection
 import org.junit.{Assert, Test}
 import org.junit.runner.RunWith
 import org.junit.runners.JUnit4
-import scala.collection.immutable.{ArraySeq, List, Range}
+
+import scala.collection.immutable.{ArraySeq, List, Range, Vector}
+import scala.language.higherKinds
 import scala.tools.testing.AssertUtil._
 
 @RunWith(classOf[JUnit4])
@@ -58,6 +60,34 @@ class IterableTest {
     Assert.assertEquals(expected, occurrences(xs))
   }
 
+  @Test
+  def sizeCompareInt(): Unit = {
+    val seq = Seq(1, 2, 3)
+    assert(seq.sizeCompare(2) > 0)
+    assert(seq.sizeCompare(3) == 0)
+    assert(seq.sizeCompare(4) < 0)
+  }
+
+  @Test
+  def sizeCompareIterable(): Unit = {
+    def check[I1[X] <: Iterable[X], I2[X] <: Iterable[X]]
+    (f1: IterableFactory[I1], f2: IterableFactory[I2]): Unit = {
+      val it = f1(1, 2, 3)
+      assert(it.sizeCompare(f2(1, 2)) > 0)
+      assert(it.sizeCompare(f2(1, 2, 3)) == 0)
+      assert(it.sizeCompare(f2(1, 2, 3, 4)) < 0)
+    }
+
+    // factories for `Seq`s with known and unknown size
+    val known: IterableFactory[IndexedSeq] = Vector
+    val unknown: IterableFactory[LinearSeq] = List
+
+    check(known, known)
+    check(known, unknown)
+    check(unknown, known)
+    check(unknown, unknown)
+  }
+
   @Test def copyToArray(): Unit = {
     def check(a: Array[Int], start: Int, end: Int) = {
       var i = 0